[Fibre Channel] The “Expansion” port within a Fibre Channel switch that connects to another Fibre Channel switch or bridge device via an inter-switch link.

E_Ports are used to link Fibre Channel switches to form a multi-switch fabric.


[Data Security] Acronym for Evaluation Assurance Level.


[General] Shorthand for Exabyte.


[General] Shorthand for Exabit.


[Standards] Acronym for European Broadcast Union.


[Storage System] [Data Communication] Acronym for Error Correcting Code.


[Energy] Heat exchanger technology used to leverage colder external air to provide data center cooling.

Dry side economizers use cooler outdoor air; wet side economizers use cooling towers or chillers.


[Standards] [Storage System] A family of SNIA standards for storage power management and related technologies.


[Data Security] Acronym for Encryption-Decryption-Encryption.


[Data Security] Short for electronic discovery.


[Fibre Channel] A buffer associated with end-to-end flow control.


[Fibre Channel] A credit scheme used to manage end-to-end flow control during the exchange of frames between two communicating devices.

effective capacity

[Storage System] The amount of data stored on a storage system, plus the amount of unused formatted capacity in that system.

There is no way to precisely predict the effective capacity of an unloaded system. This measure is normally used on systems employing space optimization technologies.

An estimated calculation may be made as follows. Let D = the size of data already stored, Fd be the formatted capacity used to store that data, and Ft be the total formatted capacity on the system. Then the estimated effective capacity Ee is given by the formula Ee = D / (Fd / Ft). No unused formatted capacity is used in the estimation calculation.

effective capacity power efficiency

[Storage System] The ratio of the effective capacity of an idle storage system to the amount of power required to maintain the system in a ready idle state.

egress routing function

[Fibre Channel] An entity within a Routing Function that performs the egress routing function role.

egress routing function role

[Fibre Channel] A process within a Routing Function that validates the frame, translates the S_ID, and then forwards the frame to the Native Fabric.


[General] Shorthand for Exbibit.


[General] Shorthand for Exbibyte.

electrical efficiency

[Computer System] The efficiency of any electrical device which transforms one type of power into another.

Efficiency is defined as output power divided by input power expressed as a percentage. All electrical components in a computer system, such as PDUs, UPSs and power supplies, incur some degree of power loss. Determining the total power loss in smaller systems with one power supply can be done by straightforward measurement of wall plug power and the total power supplied at the power supply's outputs. Larger systems require more complex methods.

electronic discovery

[Legal] Discovery that includes the identification, preservation, collection, processing, review, analysis, or production of Electronically Stored Information. [ISO/IEC 27050-1]

Although electronic discovery is often considered a legal process, its use is not limited to the legal domain. See e-discovery.

electronic storage element

[Storage System] Synonym for Solid State Disk.

Electronically Stored Information (ESI)

[Legal] Data or information of any kind and from any source, whose temporal existence is evidenced by being stored in, or on, any electronic medium. [ISO/IEC 27040]

Electronically Stored Information (ESI) includes traditional e-mail, memos, letters, spreadsheets, databases, office documents, presentations, and other electronic formats commonly found on a computer. ESI also includes system, application, and file-associated metadata (3.26) such as timestamps, revision history, file type, etc. Electronic medium can take the form of, but is not limited to, storage devices and storage elements.

embedded controller
embedded storage controller

[Storage System] An intelligent storage controller that mounts in a host computer’s housing and attaches directly to a host’s internal I/O interconnect, which is attached to storage devices mounted inside the host computer’s housing.

Embedded controllers obviate the need for host bus adapters and external host I/O interconnects. Embedded storage controllers differ from host bus adapters in that they provide functions beyond I/O interconnect protocol conversion (e.g., RAID).


[Storage Systems] SNIA Emerald™ Power Efficiency Measurement Specification or SNIA Emerald™ Program.

Emerald Specification

[Storage Systems] SNIA Emerald™ Power Efficiency Measurement Specification.

Defines a uniform taxonomy of storage subsystems and a standard way of measuring power efficiency of the storage subsystems defined in the taxonomy. For more detailed information, please consult the SNIA Emerald Program website (https://www.snia.org/emerald).


[Fibre Channel] An encapsulation header used for forwarding FC frames from a source Routing Function to a destination Routing Function.

Encapsulating Security Payload (ESP)

[Data Security] A component of IPsec that permits the specification of various confidentiality mechanisms.


[Fibre Channel] Generation of transmission characters from valid data bytes and special codes.


[Data Security] The conversion of plaintext to encrypted text with the intent that it only be accessible to authorized users who have the appropriate decryption key.

End Of Frame (EOF)

[Fibre Channel] A group of ordered sets that delineates the end of a frame.

end-to-end encryption

[Data Security] Encryption of information at its origin and decryption at its intended destination without intermediate decryption.

end-to-end flow control

1. [Network] Control of message flow between the two end parties to a communication on a network.

2. [Fibre Channel] Flow control that occurs between two communicating Fibre Channel Nx-Ports.

end-to-end security

[Data Security] Safeguarding information in an information system from point of origin to point of destination. [CNSSI-4009]

energy efficiency

[Computer System] The power efficiency of a system over time.

While power and energy efficiency look about the same to a layman, the numbers may be different (even neglecting the units) on account of temporal variations in supply voltages, power and load factors and so on.

Enhanced Transmission Selection (ETS)

[Network] A DCB component that specifies a frame scheduling mechanism to support the allocation of bandwidth amongst traffic classes that share a link.

ENode MAC address

[Fibre Channel] [Network] The MAC address used by the ENode during the FCoE Initialization Protocol (FIP).

Enterprise Resource Management (ERM)

[Management] [Network] Software that manages all aspects of an organization's assets, systems, services and functions.

ERM systems manage a set of resources in the wider perspective of an organization's entire business. Managing in an enterprise context requires that entities be named uniquely and locatable within the enterprise, that heterogeneity of platforms and services may be assumed, and that the dynamic nature of the environment is taken into account.

Enterprise Systems Connection (ESCON)

[Storage System] A 200 Mbps serial I/O interconnect used on IBM Corporation’s Enterprise System 9000 data center computers.

Similar to Fibre Channel in many respects, ESCON is based on redundant switches to which computers and storage subsystems connect using serial optical connections.


[Data Security] A measure of the amount of uncertainty that an attacker faces to determine the value of a secret. [NIST SP 800-63]

The value is sometimes measured in bits of security strength, where a value of 0 indicates no security strength (i.e., full predictability or no randomness) and a positive value indicates increasing security strength.

entry/exit slot

[Storage System] A location in a library through which a removable volume can be inserted or removed.


1. [Fibre Channel] Acronym for End of Frame.

2. [File System] A designation or marker for the end of a file.

erasure coding

[Data Recovery] A forward error correction technology used to provide data resiliency and long-term data integrity, by spreading data blocks and parity information across multiple storage devices or systems that may be in multiple physical locations.

Both the level of resiliency and where erasure coding is applied (at the array, at the node, or at the system level) can significantly affect how much processing overhead it consumes.


[Management] [Network] Acronym for Enterprise Resource Management.

Error Correcting Code (ECC)

[Storage System] [Data Communication] A scheme for checking the correctness of data that has been stored and retrieved, and correcting it if necessary.

An ECC consists of a number of bits computed as a function of the data to be protected, and appended to the data. When the data and ECC are read, the function is recomputed, the result is compared to the ECC appended to the data, and correction is performed if necessary. Error correcting codes differ from cyclic redundancy checks in that the latter can detect errors, but are not generally capable of correcting them. See cyclic redundancy check.


[Storage System] Acronym for Enterprise Systems Connection.


[Legal] Acronym for Electronically Stored Information.


[Data Security] Acronym for Encapsulating Security Payload.


[Network] A local area networking technology based on packetized transmissions between physical ports over a variety of electrical and optical media.

Ethernet can transport any of several upper layer protocols, the most popular of which is TCP/IP. Ethernet standards are maintained by the IEEE 802.3 committee.

The unqualified term Ethernet usually refers to 10 Mbps transmission on multi-point copper. Fast Ethernet is used to denote 100 Mbps transmission, also on multipoint copper facilities. Ethernet and Fast Ethernet both use CSMA/CD physical signaling. Gigabit Ethernet (abbreviated GBE) transmits at 1250 Megabaud (1 Gbit of data per second) using 8b/10b encoding with constant transmission detection.

Ethernet adapter

[Network] An adapter that connects an intelligent device to an Ethernet network., usually called an Ethernet network interface card, or Ethernet NIC.

See NIC.

European Broadcast Union (EBU)

[Standards] An alliance of public service media organizations, with members in 56 countries.

In the domains of networks and storage systems, the EBU cooperates with relevant organizations including SMPTE, VSF, AMWA and others.

eventual consistency

A behavior of a distributed system that does not provide immediate consistency guarantees.

See CAP Theorem.

evidence preservation

[Legal] A process to maintain and safeguard the integrity and/or original condition of the potential digital evidence. [ISO/IEC 1WD 27037]


[Data Recovery] Acronym for External Volume Serial Number.

Exabit (Ebit)

[General] Shorthand for 1,000,000,000,000,000,000 (1018) bits.

The SNIA uses the base 10 convention commonly found in I/O-related and scientific literature rather than the base 2 convention (1,152,921,504,606,846,976, i.e., 260) common in computer system and software literature.

See also Exbibit.

Exabyte (EB)

[General] Shorthand for 1,000,000,000,000,000,000 (1018) bytes.

The SNIA uses the base 10 convention commonly found in I/O-related and scientific literature rather than the base 2 convention (1,152,921,504,606,846,976, i.e., 260) common in computer system and software literature.

See also Exbibyte.

Exbibit (Eibit)

[General] Shorthand for 1,152,921,504,606,846,976 (260) bits.

Binary notation is most commonly used for semiconductor memory sizes.

See also Exabit.

Exbibyte (EiB)

[General] Shorthand for 1,152,921,504,606,846,976 (260) bytes.

Binary notation is most commonly used for semiconductor memory sizes.

See also Exabyte.


[Fibre Channel] A set of one or more non-concurrent related sequences passing between a pair of Fibre Channel ports.

An exchange encapsulates a “conversation” such as a SCSI task or an IP exchange. Exchanges may be bidirectional and may be short or long lived. The parties to an exchange are identified by an Originator Exchange_Identifier (OX_ID) and a Responder Exchange_Identifier (RX_ID).


[Fibre Channel] A generic term denoting either an Originator Exchange Identifier (OX_ID) or a Responder Exchange Identifier (RX_ID).

exchange status block

[Fibre Channel] A data structure that contains the state of an exchange.

An originator N_Port or NL_Port has an Originator Exchange Status Block and a Responder N_Port or NL_Port has a Responder Exchange Status Block for each concurrently active exchange.

exclusive connection

[Fibre Channel] A Class 1 dedicated connection without intermix.

expansion card
expansion module

[Computer System] A collective term for optional adapters in the form of printed circuit modules that can be added to intelligent devices.

Expansion cards include host bus adapters, network interface cards, as well as NVRAM, console, and other special purpose adapters.

expansion slot

[Computer System] A mounting and internal bus attachment device within an intelligent device into which expansion cards are inserted.

expired data

[Data Management] Data that is no longer required to be retained for any reason, becoming a candidate for permanent deletion.

Data may become expired when it has reached its defined retention period or when an event makes it obsolete and it has no further value to the organization. See disposition policy.

explicit addressing

[Storage System] A form of addressing in which the data’s address is explicitly specified in the access request.

See implicit addressing.


[Data Security] A defined way to breach the security of an IT system through a vulnerability.


1. [Computer System] Synonym for present, i.e., to cause to appear or make available.

2. [Computer System] To move objects, such as data, from within a system to a location outside the system, usually requiring a transformation during the move.

Disk array control software exports virtual disks to its host environment. In file systems, a directory may be exported or made available for access by remote clients.

extended file attribute

[File System] An element of file metadata.


[Fibre Channel] A sequence of words that may be present in a frame between the SOF delimiter and the Frame_Header to support frame handling functions not enabled by the Frame_Header.

eXtensible Access Method (XAM)

[Standards] An interface standardized by the SNIA that provides applications with standard methods for storing data and associated metadata on fixed content storage.

The XAM Application Programming Interface (API) is being standardized by SNIA.

eXtensible Markup Language (XML)

[Standards] A universal format for structured documents and data on the World Wide Web.

The World Wide Web Consortium is responsible for the XML specification. Seewww.w3.org.


1. [Storage System] A set of consecutively addressed FBA disk blocks that is allocated to consecutive addresses of a single file.

2. [Storage System] A set of consecutively located tracks on a CKD disk that is allocated to a single file.

3. [Storage System] A set of consecutively addressed disk blocks that is part of a single virtual disk-to-member disk array mapping.

A single disk may be organized into multiple extents of different sizes, and may have multiple (possibly) non-adjacent extents that are part of the same virtual disk-to-member disk array mapping. This type of extent is sometimes called a logical disk.

external controller
external disk controller
external storage controller

[Storage System] An intelligent storage controller that mounts outside its host computer’s enclosure and attaches to hosts via external I/O interconnects.

External storage controllers usually mount in the enclosure containing the disks they control.

External Volume Serial Number (EVSN)

[Data Recovery] A human-readable volume serial number on a removable volume.


[Data Communication] The center region of an eye diagram that does not occur for correctly formed signals, that distinguishes presence of signal (region above the eye) from absence of signal (region below the eye).

eye diagram

[Data Communication] A diagram used to specify optical or electrical signal transition characteristics for transmitters, in which the horizontal axis represents normalized time from pulse start and the vertical axis represents normalized amplitude.

eye opening

[Data Communication] Quantitative measure of the space in an eye diagram that does not occur for correctly formed signal transitions, and that prevents signal values from incorrectly being identified as high or low.